However, different from abnormal users above (spammers, shilling group, and fraud), professional malicious users are smarter and craftier. Shilling attacks inject fake ratings or reviews just before the recommendation process [1]}, [2]}, while for PMUs, all the actions that professional malicious users have taken are well-behaved by the rules of e-commerce websites (called masking strategies). They utilize the bug of abnormal detections, without leaving low ratings and negative feedback at the same time, to avoid detections. Then they can make illegal profits and hurt the electronic retailers. Basically, they are “normal” users for the existing abnormal user detection models, which makes the professional malicious user detection a critical issue in the recommender system area.
